If you have a certain type of self-hosted blog (see below), I recommend doing a quick Google search for various common spam terms to see if your blog has been compromised. For example, if your site were example.com, you could try this Google search:
Do the search results make your blog look like a spam site? If so, then your blog has likely been subtly compromised, even if your blog looks normal when you actually look at it.
Types of blogs: this is apparently most common in WordPress blogs, but I hear it's also cropped up in Joomla and elsewhere. I assume that it doesn't happen in blogs hosted by big organizations (such as LiveJournal, Dreamwidth, Blogger, Tumblr, etc), only in blogs installed on your site.
(Sorry for the alarmist headline; I was having a hard time balancing wanting to get the word out against not wanting to panic people.)
This has been happening to a lot of blogs lately. What's going on is that there's a hack (known as the pharma hack) that changes what search engines see when they look at a blog, without changing what normal users see. So the blogger blogs away, thinking nothing's wrong, and their readers don't see anything wrong, but search engines think the blog is a spam site.
The good news is that there's a way to fix the hack. The bad news is that it's a little technical; it involves deleting files and tweaking your database. So be sure to back stuff up before you try to follow those directions, and if you're uncertain about the technical stuff, you may want to enlist someone's help.
There's another article about the hack that's more from a Search Engine Optimization point of view, but it may provide useful info for some of you.
If you have multiple self-hosted blogs on multiple sites, it's worth checking them all.
If you want to know more about this kind of thing, you may be interested in the Fetch as Googlebot tool, which shows you a page the way that Google sees it.